There is a continuing need for functional fluids which are capable of functioning at temperature extremes such as from sub-zero temperatures to 540.degree. C. or higher. For example, synthetic lubricants for jet engines and experimental low heat rejection engines such as adiabatic engines, hydraulic fluids for supersonic aircraft and coolants for electronic equipment are required to function over this wide range of temperatures. These temperature range requirements present difficult problems of developing compositions which are liquid and thermally stable at the very high temperatures, and which remain in liquid form at low temperatures. It is also necessary to design materials which have adequate temperature-viscosity properties and lubricity and which have adequate lubricating characteristics within the entire temperature range.
Piston engines used in automobiles or generally as power sources usually have water or air-cooled cylinders in order to keep the cylinder walls cool enough to permit oil lubrication of the piston. Lubricating oil compositions primarily based upon mineral oils and including various chemical additives have been effective lubricants of the present combustion engines.
Automotive engineers, however, are developing a new generation of engines that are expected to be more powerful, use less fuel, weigh less and be smaller than existing engines. These future engines are being designed to operate at exceedingly high temperatures since it has been established that when engines run at higher temperatures, fuel efficiency increases. The high temperatures in the new engines will be attained by removing the cooling system from the engine which will also allow the engines to be smaller.
Most present-day lubricants based upon mineral hydrocarbon oils cannot withstand such high temperatures or perform satisfactorily at such high temperatures because the mineral oil decomposes or is volatile thereby leaving the movable engine parts poorly lubricated. Additionally, the decomposition of the mineral oil results in the formation of deposits. An ideal lubricating fluid for the expected high temperature or "adiabatic" engines should possess most if not all of the following characteristics: good deposit prevention low volatility, high thermal stability, good oxidative stability, satisfactory corrosion control, good wear control, satisfactory friction control, and acceptable viscometrics.
Various lubricants have been suggested in the prior art for use at temperatures of up to about 200.degree. C. or 230.degree. C. including the lubricants which have been used to lubricate moving parts of jet and turbo-jet engines. Most of the lubricants which have been suggested for use and which have been effective in lubricating jet engines have utilized high boiling synthetic oils as the base stock. Synthetic esters derived from polyhydroxy compounds and various compounds containing reactive carboxylic acid groups have been suggested as useful base oils for lubricants to be used at high temperatures such as obtained in jet engines. For example, U.S. Pat. Nos. 3,231,499; 3,340,286; 3,347,791; 4,049,563; and 4,519,927 describe the use of various synthetic esters, either alone or in combination with other materials such as synthetic ethers and silicones in high temperature lubricants. Generally, the lubricants will contain various chemicals to improve various properties including thermal stability, oxidation stability, reduced deposit formation, etc. For example, detergents and dispersants for use in synthetic ester lubricants are described in U.S. Pat. Nos. 3,231,499; 3,347,791; and 4,519,927. Alkali metal salts of carboxylic acids and hydroxyl-containing aromatic compounds are described in the '791 patent as useful detergents, and calcium stearate is an example found therein.
U.S. Pat. No. 4,519,927 describes lubricants useful at high temperatures and which comprise a mixture of an aryl alkyl silicone and a fatty acid ester of a hindered alcohol such as trimethylol propane or pentaerythritol. The patentees indicate that the lubricants, may contain other additives such as amine-, phenol-, and dithiophosphoric acid-type antioxidants, sulfonate-, phenate-, phosphonate-, and salicylate-type detergents, dispersants, sulfur/phosphorus-, and phosphate-type extreme pressure agents, and oiliness agents. Such additives are illustrated in the examples by phenothiazine, calcium sulfonate (TBN=25), calcium phenate (TBN=150), barium phosphonate (TBN=170), and tricresylphosphate. Examples of amine antioxidants described in this patent include phenyl-alpha-naphthylamine and phenothiazine.
The use of high boiling synthetic ethers as base oils for lubricants for jet engines is described in U.S. Pat. No. 2,801,968, and polyolefins such as polyalphaolefins are described as useful base stocks in high temperature lubricants in U.S. Pat. No. 3,280,031. The use of silicon fluids, either alone or in combination, as base oils for high temperature lubricants is described in, for example, U.S. Pat. Nos. 3,267,031; 3,293,180; and 4,049,563.
Published European Patent Application 0294096 describes lubricants based on natural or synthetic basestocks which contain a high molecular weight carboxylic dispersant and a metal detergent which may be a neutral or basic sulfurized alkyl phenol. The lubricants may contain other additives such as antioxidants. Examples of antioxidants include calcium nonyl phenol sulfide, dioctyldiphenyl amine and phenyl alpha-naphthyl amine.
WO 87/01722 describes diesel lubricants containing a natural or synthetic basestock containing a carboxylic derivative dispersant and a basic alkali metal salt. The lubricants may contain other additives such as metal dithiophosphates, various detergents including metal carboxylates, sulfonates and phenates, and antioxidants. One example of a metal detergent is a basic calcium salt of a sulfurized tetrapropenyl phenol, and an alkylated aromatic amine is also included in the oil.
High temperature jet lubricants are described in U.S. Pat. No. 3,247,111 which comprise a major proportion of a synthetic ester, minor amounts of various additives including antioxidants which include amines, phenols, esters, phosphites, etc. Examples of antioxidants described in this patent include diaromatic amines such as dinaphthyl amine, and hindered phenols such as 2,4-di-tertiarybutyl p-cresol, etc. Combinations of different diaromatic amines are described as being preferred.
U.S. Pat. No. 3,278,436 describes lubricants containing certain melamine derivatives as an essential lubricating ingredient in combination with other lubricants which include synthetic esters. Antioxidants are also included in the lubricating compositions to hinder the auto oxidation which occurs at temperatures above 150.degree. C. Cyclic aromatic amines and hydroxy-substituted aromatics are described as useful antioxidants. Of the antioxidants in the class of hydroxyl-substituted aromatics, hindered phenols such as 2,6-di-tert-butyl-4-ethyl phenol and methylene coupled hindered phenols such as 2,2'-methylene-bis-(4-methyl-6-tert butyl phenyl) are identified. Synthetic ester lubricants also containing antioxidants which may be aromatic amines or of the phenolic type are also described in U.S. Pat. No. 3,673,226. Synthetic ester-based gas turbine lubricants containing diaromatic amines and methylene coupled phenols such as 4,4'-methylene-bis(2,6-di-t-butyl phenyl) are described in U.S. Pat. No. 3,912,640. The base stock utilized in the preparation of these lubricants comprise a blend of a synthetic ester and a low viscosity mineral oil. The amount of mineral oil may range from about 20 to about 80% of the base stock.